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Melanophilin directly links Rab27a and myosin Va through its distinct coiled-coil regions
Kazuaki Nagashima1, Seiji Torii, Zhaohong Yi
1Laboratory of Gene Engineering, Institute for Molecular and Cellular Regulation, Gunma University, 3-39-15 Showa-machi, Maebashi, Gunma, Japan.
Abstract:
Rab GTPases regulate the membrane transport pathways by recruiting their specific effector proteins. Melanophilin, a putative Rab effector, has recently been identified as a gene that is mutated in leaden mice, in which peripheral localization of melanosomes is impaired in melanocytes. Genetic studies suggest that three coat-color mutation genes, dilute (MyoVa(d)), ashen (Rab27a(ash)), and leaden (Mlph(ln)), act in the same or overlapping pathways. Here we have cloned and characterized a human melanophilin homolog, which belongs to the rabphilin3/granuphilin-like Rab effector family. Cosedimentation assays using recombinant proteins reveal that melanophilin directly binds to Rab27a and myosin Va through its N-terminal and its first C-terminal coiled-coil region, respectively. Moreover, we show that Rab27a, melanophilin, and myosin Va form a ternary complex in the human melanocyte cell line HMV-II. These findings suggest that melanophilin has a role in bridging Rab27a on melanosomes and myosin Va on actin filaments during melanosome transport. We also propose that the Rab-binding region conserved in a novel rabphilin3/granuphilin-like Rab effector family constitutes an alpha-helix-based coiled-coil structure.
Insights
Melanophilin bridges Rab27a on melanosomes and myosin Va on actin filaments, crucial for melanosome transport. This finding clarifies the molecular mechanism underlying melanosome localization defects.
Area of Science:
- Cell biology
- Molecular genetics
- Protein interactions
Background:
- Rab GTPases are key regulators of intracellular membrane transport.
- Melanophilin mutations impair melanosome localization in melanocytes.
- Coat-color genes like dilute, ashen, and leaden suggest overlapping pathways in melanosome transport.
Purpose of the Study:
- To clone and characterize the human melanophilin homolog.
- To elucidate the molecular interactions of melanophilin in melanosome transport.
- To understand the role of melanophilin in bridging Rab27a and myosin Va.
Main Methods:
- Cloning and characterization of human melanophilin.
- Cosedimentation assays with recombinant proteins.
- Analysis of protein complex formation in human melanocyte cell lines.
Main Results:
- Human melanophilin directly binds Rab27a and myosin Va.
- Rab27a, melanophilin, and myosin Va form a ternary complex.
- Melanophilin acts as a bridge between Rab27a on melanosomes and myosin Va on actin filaments.
Conclusions:
- Melanophilin is essential for linking melanosomes to the actin cytoskeleton via myosin Va.
- This interaction is critical for effective melanosome transport.
- The Rab-binding region of this novel effector family forms a coiled-coil structure.